1. Field of the Invention
The present invention relates to a helical broach for roughing.
2. Description of the Related Art
Heretofore, roughing and finishing using helical broaches have been a mainstream in manufacture of internal gears. Helical broaches each perform cutting on a prepared hole formed in advance in a work piece by use of its cutting teeth which are larger gradually and sequentially in an outer radial direction and in a tooth thickness direction. Various studies have been made on helical broaches. There exist many variations of helical broaches such as: a type which includes a set of several separate units for roughing and finishing; a type in which a roughing cutting tooth group and a finishing cutting tooth group are put together into one unit; as well as an assembly type in which a roughing cutting tooth group and a finishing cutting tooth group are assembled and integrated together.
In machining using these helical broaches, it has been known that a helix angle causes a difference in cutting performance between the left side and the right side (an acute angle portion side and an obtuse angle portion side) of a toothed gear, which may in turn lead to unbalanced cutting in the right and left sides of the toothed gear.
Since the cutting teeth are arranged at the helix angle, the angle between a rake face and one tooth face of a cutting tooth is acute while the angle between the rake face and the other tooth face is obtuse. This angular difference between both sides of the rake face causes the unbalanced cutting in machining using the helical broach and thus is one of the causes hindering improvement of the accuracy of machining using the helical broach. The cutting performance on the acute angle portion side is finer than that on the obtuse angle portion side, particularly in a case of a helical broach of an axially-perpendicular gullet type configured by arranging multiple spline teeth perpendicularly to the axial direction. In other words, since the cutting performance on the obtuse angle portion side is lower than the cutting performance on the acute angle portion side, there occurs a large difference in cutting performance between the tooth faces on both sides of the rake face. This causes a problem of affecting the tooth profile accuracy and the tooth trace accuracy.
To address such problem, Patent Document 1 below discloses a configuration in which a relief angle at an obtuse corner of a cutting tooth is made larger than a relief angle at an acute corner of the cutting tooth to thereby balance the cutting performances of both of the corner portions. The cutting performance is thus balanced between the right and left cutting tooth corner portions of each cutting tooth of spline teeth.